1. Field of the Invention
The present invention relates to a sheet feeding apparatus for separating sheets stacked on a sheet stacking portion one by one and for feeding the separated sheet to an image forming apparatus.
2. Related Background Art
Among conventional image forming apparatus such as printers, copying machines, facsimiles and the like, there is an image forming apparatus in which an image is formed on a thick sheet such as a post card and an envelope or a special sheet such as a plastic thin plate, as well as a plain paper. In such an image forming apparatus, the feeding of the sheet to an image forming portion is effected by manual sheet insertion one by one or is effected by a sheet feeding apparatus automatically and continuously.
FIG. 10 shows a schematic construction of a printer as an example of an image forming apparatus having such a sheet feeding apparatus. In FIG. 10, a sheet P fed by a sheet feeding apparatus 1A for feeding sheets to printing means 4 one by one is conveyed while being pinched between conveying rollers 2, 3, and, after printing is effected by the printing means 4, the sheet is discharged out of the apparatus by discharge rollers 5, 6.
Such a sheet feeding apparatus 1A is disclosed in Japanese Utility Model Publication No. 8-3396, for example, and FIG. 11 shows a construction of such an apparatus.
In FIG. 11, a sheet feeding roller 10 having a D-shaped cross-section has a cylindrical portion 10a and a straight portion 10b. Incidentally, a shaft 10c of the sheet feeding roller 10 is supported by a guide block 11. Further, a cassette 12 has therein a stacking plate 12a on which a plurality of sheets are set in a stacked condition. By a biasing force of a spring 12c for biasing the stacking plate 12a, a sheet stack P stacked (rested) on the stacking plate is biased toward the sheet feeding roller 10.
A separation pad 13 attached to a bracket 13a is disposed in a rotational movement path of the cylindrical portion 10a of the sheet feeding roller 10 and is biased the shaft 10c of the sheet feeding roller along a guide 15 by means of a spring 14.
An idle (idler) roller 16 is rotatably attached to the guide block 11, and a movable idle roller 17 is attached in an elongated groove 11a of the guide block 11 via a shaft 17a for shifting movement. The movable idle roller 17 is biased toward the separation pad 13 by a spring 18 to abut against the separation pad 13. Incidentally, a biasing force F2 of the spring 18 is selected to be smaller than a biasing force F1 of the spring 14 of the separation pad 13 (i.e., F1&gt;F2).
Further, when it is assumed that a friction force between the cylindrical portion 10a of the sheet feeding roller 10 and an uppermost sheet P1 in the sheet stack is f1, a friction force between a next sheet P2 and the separation pad 13 is f2, and a friction force between the uppermost sheet P1 and the next sheet P2 is f3, coefficients of friction of the sheet feeding roller 10 and the separation pad 13 are selected to satisfy the following relationship: EQU f1&gt;f2&gt;f3.
Now, a sheet feeding operation of the sheet feeding apparatus 1A will be explained.
In a waiting condition, as shown in FIG. 11, the straight portion 10b of the sheet feeding roller 10 is opposed to the sheet stack P so that the sheet feeding roller 10 is not contacted with the sheet P. Further, since the biasing force F2 of the spring 18 of the movable idle roller 17 is smaller than the biasing force F1 of the spring 14 of the separation pad 13, the movable idle roller 17 is pushed upwardly by the separation pad 13 so that the shaft 17a abut against an upper end of the elongated groove 11a.
When the sheet feeding operation is started, the sheet feeding roller 10 is rotated in a direction shown by the arrow, and, by contacting the circular portion 10a with the uppermost sheet P1 in the sheet stack P, the uppermost sheet P1 is sent toward the separation pad 13. In this case, due to the friction force between the sheet P1 and the next sheet P2, the next sheet P2 may be sent together with the uppermost sheet P1. However, the next sheet P2 is separated from the uppermost sheet P1 by the separation pad 13 in the following manner, and only the uppermost sheet P1 is fed.
When a leading end of the next sheet P2 abut against the separation pad 13, the movement of the next sheet is prevented to primarily separate the next sheet from the uppermost sheet P1. Further, as mentioned above, since the friction force f1 between the cylindrical portion 10a of the sheet feeding roller 10 and the uppermost sheet P1, the friction force f2 between the next sheet P2 and the separation pad 13 and the friction force f3 between the uppermost sheet P1 and the next sheet P2 are selected to satisfy the relationship f1&gt;f2&gt;f3, as the sheet feeding roller is rotated, when both the uppermost sheet P1 and the next sheet P2 are pinched between the cylindrical portion 10a of the sheet feeding roller 10 and the separation pad 13, the next sheet P2 a prevented from being moved by the friction force f2 between the separation pad 13 and the next sheet and is secondarily separated from the uppermost sheet P1, with the result that only the uppermost sheet P1 is fed.
Thereafter, when the sheet feeding roller 10 is further rotated, the sheet feeding roller 10 is returned to the waiting condition where the separation pad 13 is not urged by the cylindrical portion 10a, as shown in FIG. 11. Incidentally, in this case, due to the difference between the biasing forces F1, F2 of the springs 14, 18, a separation pad 13 is stopped in a condition that the movable idle roller 17 is pushed upwardly until the shaft 17a abuts against the upper end of the elongated groove 11a.
By the way, since the separation pad 13 is disposed in the rotational movement path of the cylindrical portion 10a of the sheet feeding roller 10, as the cylindrical portion 10a is rotated, the separation pad 13 is pushed downwardly by the cylindrical portion 10a. Since the movable idle roller 17 is biased toward the separation pad 13 by the spring 18, even when the separation pad 13 is pushed downwardly, the movable idle roller abuts against the separation pad 13, and this abutment aids the separating operation.
Further, by adopting the arrangement in which the movable idle roller 17 is biased toward the separation pad 13 to pinch the sheet P1 between the movable idle roller and the separation pad 13, even when the friction force acts between the uppermost sheet P1 and the next sheet P2, while the sheet P1 is being conveyed by the conveying rollers 2, 3, the next sheet P2 can be prevented from being sent together with the uppermost sheet P1.
By the way, when such a conventional sheet feeding apparatus is used in the printer as shown in FIG. 10, the fed sheet P1 is conveyed while being pinched between the conveying rollers 2, 3 and is printed by the printing means 4. In this case, when the conveyance of the sheet P1 by the conveying rollers 2, 3 is started, normally, the sheet P1 is not completely left from the sheet feeding apparatus 1A. That is to say, a trailing end portion of the sheet P1 is pinched between the separation pad 13 and the movable idle roller 17, and the sheet P1 is conveyed by the conveying rollers 2, 3 in this condition.
Accordingly, the sheet P1 is conveyed by the conveying rollers 2, 3 in a condition that the sheet is subjected to load due to the pinching (in a condition that the load acts on the trailing end of the sheet) until the trailing end of the sheet leaves the nip between the separation pad 13 and the movable idle roller 17.
As mentioned above, since the separation pad 13 serves to prevent double-feeding by pinching the sheet P between the separation pad and the cylindrical portion 10a of the sheet feeding roller 10, the biasing force F1 is required to be set relatively greatly. However, when the biasing force F1 is so set, the load acting on the sheet P1 in the pinched condition becomes great.
When the load acting on the sheet P1 in the pinched condition becomes great, if a conveying force sufficient to overcome this load is not obtained by the conveying rollers 2, 3, since the sheet feeding accuracy is reduced, the printing accuracy of the printing means 4 is also reduced.
Incidentally, in order to obtain the conveying force sufficient to overcome this load, for example, if the sheet pinching force between the conveying rollers 2, 3 is increased and a driving force for driving the conveying rollers 2, 3 is increased, the entire apparatus will be made bulky and/or power consumption will be increased. Further, the wear of the conveying rollers 2, 3 will be increased.